The present invention relates to a surface emitting semiconductor laser device.
FIG. 1 shows a sectional view of a conventional surface-emitting semiconductor laser device disclosed in pages 930 to 932 of the journal "Applied Physics Letters"vol. 46, No. 10, 1985. The device comprises a first electrode 1, a substrate 2 such as an n-type InP substrate, a buffer layer 3 such as an n-type GaInAsP. buffer layer, a first clad layer 4 such as an n-type InP clad layer, an active layer 5 such as a p-type GaInAsP active layer which has a thickness of several microns, a second clad layer 9 such as a p-type InP clad layer, reflectors 14 and 15, an insulating film 16 of SiO.sub.2 or the like, an opening 17 provided on the top of the device, and a second electrode 18. The substrate 2 and the buffer layer 3 are interposed between the first electrode 1 and the first clad layer 4. The opening 17 is provided over the first clad layer 4 to take out a laser beam from the top of the device.
The operation of the device will be described. When an electrical current is caused to flow between the first and the second electrodes 1 and 18, light emerges from the active layer 5 and is reflected by the upper and the lower reflectors 14 and 15 so that Fabry-Perot laser oscillation arises in a resonator constituted by the reflectors. The laser beam is taken out through the opening 17 provided on the top of the device. The thickness of the active layer 5 is about several microns, and the length of the resonator constituted by the reflectors 14 and 15 is also about several microns.
Since the conventional surface-emitting semiconductor laser device shown in FIG. 1 is constituted as described above, the length of the resonator is very short and the loss in the resonator is very large. When the reflectivity R of the upper and the lower reflectors 14 and 15 is 70% and the length L of the resonator is 7.mu., for example, the loss in the resonator is about 450 cm.sup.-1. For that reason, the threshold current of the device is so high that the device cannot be normally used. Also, since the construction around the reflector 15 included in the device is complicated and the length of the resonator is short, the process of manufacturing of the device is complicated.
FIG. 2 shows a cutaway perspective view of another conventional surface-emitting semiconductor laser device disclosed in "GaInAsP/InP integrated laser with butt jointed built-in distributed-Bragg-reflector waveguide" by Y. Abe, K. Kishino, Y. Suematsu and S. Arai in the journal "Electronics Letters" vol. 17, Nos. 25/26, pages 945 to 947, December 1981. The device comprises an active region 59, a Bragg reflection region 60, an n-type electrode 62, an n-type InP substrate 63, a single active layer 64 of InGaAsP and about 0.1.mu. in thickness, a clad layer 65 of p-type InP, a contact layer 66 of p-type InGaAsP, an insulating film 67 of SiO.sub.2, a p-type electrode 68, an optical waveguide layer 69 of InGaAsP, a clad layer 70 of InP, an embedded layer 71 of p-type InP, an embedded layer 72 of n-type InP, and a diffraction grating 73. The gap wavelength .lambda..sub.ga of the active layer 64 is about 1.62.mu., while that .lambda..sub.ga of the optical waveguide layer 69 is about 1.35.mu..
The operation of the device will be described. When an electrical current is caused to flow between the electrodes 62 and 68 in the active region 59, light is amplified by the active layer 64. The gap wavelength .lambda..sub.ga of the optical waveguide layer 69 is smaller than that .lambda..sub.ga of the active layer 64, that is, the optical waveguide layer is made of such a substance that the optical waveguide layer does not absorb the light at the laser oscillation wavelength. Therefore, the light is transmitted in the optical waveguide layer 69 without being absorbed and is reflected by the diffraction grating 73, so that the light is reciprocated between the Bragg reflection region 60 and the plane of cleavage of the active region 59 while being amplified by the active region. As a result, laser oscillation is caused. Since the diffraction grating 73 is of wavelength selectivity, the laser oscillation is caused in a stable single mode.
Since the conventional surface-emitting semiconductor laser device shown in FIG. 2 is constituted as described above, when the device is manufactured, the layers except the electrodes in the active region 59 are provided on the substrate 63 by crystal growth, a region corresponding to the Bragg reflection region 60 is thereafter removed by etching or the like and the optical waveguide layer 69 and the clad layer 70 are provided in the removed region by crystal growth. For that reason, at least two steps of crystal growth are needed, so that the process of the manufacturing is complicated and the yield thereof is low. Further, a loss of light at the joint of the active region 59 and the Bragg reflection region 60 is large.